In the SO2-ethanol-water (SEW) pulping process, lignocellulosic feed stocks may be fractionated into their main components (cellulose, hemicelluloses and lignin) using a 55% (v/v) ethanol-water solution in which SO2 is dissolved without any base (Na, Mg, Ca, etc.) present. The presence of SO2 leads to dissolution and hydrolysis of hemicelluloses in high yield, while the lignin becomes soluble through sulfonation. The presence of ethanol leads to rapid penetration of the biomass, thereby eliminating the long heat-up time required in the acid sulphite process. Lignin is recovered by precipitation after ethanol evaporation. The absence of a base allows recovery of unreacted SO2 by evaporation and steam stripping, while the low boiling point and low specific heat of ethanol (as compared to water) leads to energy efficient recovery of the pulping liquid by evaporation (Rakkolainen et al., 2009). Previous research in our laboratory has established that the SO2-ethanol-water (SEW) fractionation method is capable of pulping both softwoods and hardwoods efficiently at moderate conditions (Iakovlev et al. 2011, Rakkolainen et al. 2010). The SEW pulping chemistry is employed by American Process Inc. in a patent pending biorefinery process termed AVAP™ to produce cellulosic fibers, chemicals and biofuels (Retsina and Pylkkänen, 2007). American Process Inc. also has various patent applications and patents for the SEW pulping chemistry, e.g. WO 2007/146245, WO 2010/151536, WO 2011/044378 and U.S. Pat. No. 8,030,039, disclosing variations of the process. The goal of the process is to treat lignocellulosic materials for producing alcohol and other bioproducts.
Acetone, butanol, and ethanol (ABE) may be obtained by fermentation of sugars using Clostridia bacteria (Dürre, 2008). In order to utilize the dissolved hemicellulose sugars in SEW spent liquor further processing and conditioning of the solution is required to allow fermentation by microorganisms. For example, the pH of the SEW spent liquor is very low (about 1.0) and adjustment to a neutral level is necessary to avoid harming the bacteria. Furthermore, fermentation inhibitors such as SO2, ethanol, lignin, formic acid and furanic compounds, must be totally removed (Sklavounos and van Heiningen, 2010). It was shown that SO2 is inhibitory to Clostridia bacteria at concentrations as low as 10-50 ppm, while formic acid and furfural are not tolerable at levels above 0.5 and 1.0 g L−1, respectively (inhibition tests performed by Teräsvuori et al., 2010). In addition to fermentability of the final liquor, an efficient recovery of the cooking chemicals, i.e. SO2 and ethanol, is required for the viability of the process. So far, it has not been possible to remove the above-mentioned inhibitors to that extent. Therefore, a constant need exists for an improved method for obtaining suitable liquor for ABE fermentation.